OBJECTIVE Strong evidence has suggested that there is a circadian periodicity of acute coronary event. Beta-blockers, aspirin and angiotensin-converting enzyme inhibitors decrease the rate of acute myocardial infarction (AMI) and blunt the peak incidence in the morning. However, such effect has not been evaluated for statins. Accordingly, the present study aimed to evaluate the influence of statin therapy on circadian variation of AMI. METHODS This retrospective study consisted of 451 consecutive patients with acute ST segment elevation AMI. The patients were divided into two group based on prior statin usage. In statistical analysis t test, Chi-square test and Mann Whitney U test were used for comparison of groups. We used harmonic regression models to evaluate the circadian variation of onset of MI symptoms in patients receiving statin and patients not receiving statin. RESULTS In all study participants, the highest incidence of AMI was between 6.00 and 12.00; the odds ratio was 1.34 (95% CI 1.20 to 1.46, p=0.001). In the non-statin group, the highest incidence of AMI occurred between 0:00 A.M. and 06.00. There was still a peak incidence between 6.00 A.M. and noon in the statin therapy receiving group; the odds ratio was 1.61 (95% CI 1.34 to 1.80, p=0.001). Accordingly, there was no statistical difference between the statin and non-statin groups regarding circadian variation of AMI. Prior usage of statin did not blunt the peak incidence of AMI in the morning. CONCLUSIONS Prior usage of statin does not seem to play a role in the circadian periodicity of AMI.